The Source engine is a 3D game engine developed by Valve Corporation. Its unique features include a large degree of modularity and flexibility, an artist-driven, shader-based renderer, accurate lip sync and facial expression technology, and a powerful, efficient and completely network-enabled physics system.
The Source Engine is an outdated piece of **** that makes you spend 60% of your time trying to find alternative ways to make something because of the amazing number of bugs the SDK contains.
I want to make a laser that dissolves any NPCs ; I can't because the beam doesn't do any damages even if I set the properties/flags rightly. So I try to parent a trigger_hurt to the beam but even if I set "Damage Type" to "Beam", it is only turning a NPC to a ragdoll and doesn't dissolve it as I wanted. Great, now I have laser that just turn without blood a combine into a ragdoll, very realistic !
I want to make an introduction sequence with a point_viewcontrol that will fly over a path_corner/path_track, I can't because the viewcontrol doesn't stop and still continue to fly in a direction even when it's out of the map. Great, I have to use it in a static way.
There are so many stupid bugs that force you to change things you wanted in your mod.
And please don't tell me "wfgo I make css map and soucre 4 ever !!1!1 never had problems", since the Source Engine may seems nice only when you make what Valve is making now.
This engine has a lot of pros and cons. One one hand, it's very easy to get into and it can look pretty damn good in the right hands. But on the other hand, you got a 7 year old tool set with mostly command line tools that Valve breaks when ever they get bored. And depending on the version of Source you use(2006, 2007, 2009(no code), AlienSwarm, Portal2(no code)), you might get stuck with half working features(like shadow mapping in 2007).
The main pro is; the fact that it comes with an entire game(2006/2007).
Main con is; half working tools that love to crap out for no apparent reason.
If the tools were in decent shape, and if they would release more fully working(AS has no shader support) codebases. Then I would give it a 10/10 and wouldn't be in the process of learning the UDK.